Mufflers, resonators and the like are provided along the exhaust lines of internal combustion engines to reduce noise emitted from combustion in the engine cylinders. A muffler typically defines an elongated path for exhaust gas that passes through or is bordered by a volume of packing material that attenuates sound vibrations. A simple example is a cylinder containing packing material traversed by a longitudinal tube for gas passage, with perforations in the tube exposing the gas path to the packing disposed around the tube. In other configurations, the path might define a circuitous route, and encounter baffles or screens. Attention is paid when structuring a muffler and its packing material to achieve an appealing or distinctive engine sound.
The packing fill in and around the exhaust gas path comprises resilient forms, typically thin fibers, that are resiliently displaceable with gas pressure variations at audible frequencies, thereby damping the amplitude of sounds emitted through the exhaust system. The fill needs to be confined in the muffler in one way or another so as not to become entrained in the exhaust flow and ejected. The fill is preferably not subject to chemical corrosion or combustion when subjected to the exhaust gas.
One way to confine muffler packing material and to retain a shape for the packing is to include an adherent binder so that packing fill fibers or other bodies adhere to one another where they come into contact. Such binders are typically oxidized, combusted and otherwise broken down in hot gas the environment of the exhaust. Furthermore, binders that hold the packing in a fixed shape during muffler assembly, and are intended to burn away in the hot exhaust, tend to smoke and smell during a muffler break-in period.
To endure with adequate longevity along the exhaust path, the fill material needs to be durable and chemically unreactive. To attenuate noise, the fill needs to be resilient and arranged as a porous mass in which acoustic waves can propagate, while filling out one or more shaped voids within the muffler. An apt material for muffler fill is fiberglass. Provisions are advantageous to conform and confine the fiberglass to the shape of the void in the muffler, at least during assembly of the muffler parts. Assembly typically involves welding or swaging of muffler housing parts at seams. Loose or stray packing fibers that become lodged between the housing parts at seams can interfere with obtaining a leak-proof continuous seam.
Fiberglass or other unreactive fiber fill materials can be coated with an adherent binder and compressed in a mold, such that the binder attaches fibers together where they come into contact, holding the mass of fibers in a shape. The molded shape preferably eliminates stray fibers by fixing the relative positions of the fibers temporarily during muffler assembly. After assembly, packing is confined by the muffler structures forming the void for the packing. The binder burns away in the hot engine exhaust during an initial break-in phase of the muffler.
U.S. Pat. No. 6,068,082, dated May 2, 2000 (hereby incorporated by reference in its entirety), discloses a technique wherein continuous strand fiberglass roving is used as a muffler fill material. The continuous strand fiberglass roving is packed into an envelope of plastic mesh, heat sealed at envelope seams. The packed fill expands the envelope to complement the shape of the void in the muffler. The packed envelope is placed in the muffler packing void during muffler assembly. The open plastic mesh of the envelope confines the fiberglass roving and does not interfere substantially with movement of exhaust gas. Thus the envelope has little or no effect on the sound of the muffler. The sound of the muffler remains the same through the break-in period when the plastic mesh envelope is consumed and burns away. However, the plastic mesh produces smoke and odor as it burns away. The volume of plastic contained in a plastic mesh envelope confining uncoated fiberglass roving fill, and the surface area of the plastic, are smaller than is the case where adherent coating is applied to all the surfaces of strands of packing fill, which limits the amount of smoke and odor emitted during break-in. But it would be advantageous if the smoke and odor could be further reduced.
It should be appreciated in this disclosure that the concept of continuous strand fiberglass roving is meant to encompass variations in which the strands are single strands or multi-fiber yarns. Further, the strands or yarns need not extend continuously between extreme ends of a given run of structure (such as a filled muffler) to be deemed continuous. A continuous strand or yarn can comprise two or more lengths that are lengthwise adjacent, adjacent around a gap, or perhaps overlap one another for a distance along their lengths. This construction is generally consistent with industry usage where “continuous strand” denotes elongated fibers, often used in alignment to form yarns or fabrics, or handled in alignment with one another during filling as with continuous strand roving. Continuous strand roving is distinct from “chop strand,” the latter referring to fibers that are relatively short and typically are handled or used in random fiber alignment. Either continuous strand or chop strand can form a packing mass, a batt or a compressed non-woven mat that may be regarded as a fabric. However continuous strand but not chop strand is capable of formation into a weave or knit wherein the fabric has regularly oriented fibers spaced by gaps so as to provide a mesh, and capable of being stuffed or packed to fill out a confined space without the need to adhere the fibers to one another.
Continuous strand fiberglass roving tends to distribute itself fairly evenly in a confined space. The density of the packing can be controlled by the length of the roving that is run into the confined space or packing envelope using nozzle-like injectors that blow in a plurality of strands from spools. The fiberglass is non-reactive and produces neither smoke nor odor during its break-in period. Nor is there a change in muffler sound.
The housing of an assembled muffler forms a substantially closed void that might confine loose fiberglass roving if the roving could be introduced into the void. But it may be difficult to inject and/or introduce continuous strand fiberglass roving inside a pre-assembled muffler, although “direct fill” into an assembled muffler housing may be possible if the void for the packing is accessible and the void for the packing has a regular geometric shape. For more complicated shapes such as automobile mufflers with horizontal dividing panels, the voids that need to be filled with packing are relatively inaccessible.
One might attempt to assemble a muffler housing around a loose mass of uncompressed fiberglass roving, so as to compress the mass of roving as part of the process of bringing muffler housing parts together and thereby pack and fill the void that the parts close around. That process is difficult to accomplish, at least without stray packing fibers tending to interfere with making a continuous seam along the housing parts.
For these reasons, a confining envelope of lightweight wide-gap mesh is useful. However smoke and/or odor are associated with consumption of the mesh if it is made of plastic, even if the mesh fiber portions are thin and the gaps are wide. There is a limit to how wide the openings of a plastic mesh can be made, and how thin the mesh strands can be while being heat-sealable effectively, and otherwise made strong enough to form a fixed shape that confines compressed fiberglass packing and does not tear open at seams or permit packing strands to protrude from mesh gaps. It would be advantageous to provide improvements that provide a robust envelope and employ even less consumable material.